1. Field of the Invention
This invention relates to a drive system of a display device, particularly to such a system having a drive circuit using a current programming method.
2. Description of the Related Art
In recent years, organic electroluminescent (hereafter, referred to as EL) display device using organic EL elements have been receiving attention as a display device substituting for a CRT or an LCD. Particularly, an active matrix type organic EL display device having thin film transistors as switching elements for driving the organic EL elements has been developed. Different from LCDs, such organic EL elements are self-light-emitting elements providing luminance corresponding to a current flowing in the EL elements.
There are various types of drive systems for such an organic EL display device, and one of these is a current programming method. In this method, for obtaining luminance corresponding to a digital display signal, by utilizing such current and luminance correspondence characteristics of the organic EL element described above, a current value corresponding to the digital display signal is set by a current conversion circuit (also called a current DAC) and the current is supplied from the current conversion circuit to each of the pixels.
Particularly, in a high-precision organic EL display device, a plurality of the current conversion circuits are provided for each of pixel groups divided in columns in order to secure time for programming the current to the pixel. Such a drive system is called a multi-channel current DAC method since a channel is provided in each of the pixel groups divided in columns.
FIG. 4 is a block diagram showing a drive system of an organic EL display device of a conventional art. A plurality of pixels P11, P12 . . . each having an organic EL element is disposed in a matrix of m rows and n columns. The n pieces of current conversion circuits DAC1 to DACn are disposed for the pixel groups divided in columns, respectively. These current conversion circuits DAC1 to DACn convert digital display signals D1 to Dn inputted therein into currents I1 to In having current values corresponding to the signals D1 to Dn, respectively, and supplies the currents I1 to In to the pixel groups divided in columns, respectively.
For example, during the first horizontal scanning period, the currents I1, I2, . . . and In are supplied to the pixels P11, P12, . . . and P1n, in this order. Then, during the next horizontal scanning period, the currents I1, I2, . . . and In are supplied to the pixels P21, P22, . . . and P2n, in this order, respectively. Such a horizontal scanning is repeated to the whole remaining lines, thereby completing one field scanning period.
FIG. 5 is a table showing a correspondence relationship between the pixel groups divided in columns and the current conversion circuits DAC1 to DACn for driving these pixel groups in this drive system of the organic EL display device. As seen in FIG. 5, the pixels in each of the pixel groups divided in columns are driven by the same current conversion circuit. For example, in an n-th field, the pixels of the pixel group in the first column are driven by the current conversion circuit DAC1 indicated by “1” in FIG. 5, and the pixels of the pixel group in the second column are driven by the current conversion circuit DAC2 indicated by “2” in FIG. 5. The correspondence relationship is the same in an n+1 field and an n+2 field. The relating technology is disclosed in the Japanese Patent Application Publication No. 2003-150118.
Generally, n pieces of the current conversion circuits DAC1 to DACn are formed of LSIs, and there occurs variation in output current values of n pieces of the current conversion circuits DAC1 to DACn due to manufacture variations. This variation in the output current directly causes variations in luminance of the organic EL elements as current drive elements.
In the drive system of the display device of the conventional art shown in FIG. 4, the pixels of the pixel group in each of the columns are driven by the same current conversion circuit all the time. Therefore, when the value of the output current of the current conversion circuit provided for a certain column is unusually too high or too low compared with others, an uneven display with bright and dark parts appears in the line corresponding to the pixel group in that column.
Generally, human eyes can not recognize such an uneven display if variation of luminance is 1% or less, but it is difficult to keep the variation at 1% or less by current LSI manufacturing technologies.